The present invention relates to an automatic transmission including an auxiliary gearing, and more particularly to a hydraulic control for quick shift to engine braking or reverse drive in such an automatic transmission.
JP 59-113351 A discloses an automatic transmission including an auxiliary gearing drivingly connected in series to a main gearing.
In this known automatic transmission, the auxiliary gearing is shiftable between a high gear position and a low gear position. It includes a direct clutch as a first frictional unit to be engaged in the high gear position, and a reduction brake as a second frictional unit to be engaged in the low gear position. The high gear position is established in the auxiliary gearing when the direct clutch is engaged with the reduction brake released, while the low gear position is established in the auxiliary gearing when the direct clutch is released with the reduction brake engaged. The auxiliary gearing provides a direct drive in the high gear position, while it provides a reduction gear in the low gear position. The auxiliary gearing also includes a one-way clutch which becomes operative to serve as a substitute for the reduction brake during engine driving in the forward drive. During a shift in the auxiliary gearing from the low to high gear position by disengaging or releasing the reduction brake and engaging the direct clutch, the one-way clutch anchors a rotary member which is to be gripped by the direct clutch. This prevents excessive wear of the direct clutch and reduces a shock taking place during engagement of the direct clutch.
During engine drive running in the forward drive, the one-way clutch accomplishes the same function as the reduction brake does, so it is not necessary to keep the reduction brake engaged. Therefore, it is during engine brake running in the forward drive or during running in the reverse drive that the reduction brake has to be engaged.
If, in order to condition the auxiliary gearing in the low gear position during engine drive forward running, the reduction brake is released, there has to be set an additional control mode where both the reduction brake and direct clutch are disengaged or released in addition to two control modes, namely, a control mode where the direct clutch is engaged with the reduction brake released to condition the auxiliary gearing in the high gear position, and a control mode where the reduction brake is engaged with the direct clutch released to condition the auxiliary gearing in the low gear position during the engine brake forward running. In order to make a switch among the above-mentioned three control modes, a plurality of valves are required for shifting the auxiliary gearing between the high and low gear positions, resulting in a complicated hydraulic control system.
In order to reduce the control modes, in number, it has been the common practice to engage the reduction brake to condition the auxiliary gearing in the low gear position not only during engine brake running, but also during the engine drive forward running.
An example embodying this common practice is described referring to FIG. 5. In FIG. 5, a shift valve SFV, a reduction brake RD/B, a direct clutch D/C, one-way orifices OWO1 and OWO2, and accumulators ACC1 and ACC2 are illustrated. The shift valve SFV assumes one position when a low gear position command pressure signal P.sub.LS is supplied thereto. In this position of the shift valve SFV, a line pressure P.sub.L is allowed to cause engagement of the reduction brake RD/B, while the direct clutch D/C is allowed to communicate with a drain port and released, conditioning the auxiliary gearing in the low gear position. The shift valve SFV switches and assumes the opposite position when a high gear position command pressure signal P.sub.HS is supplied thereto. In this opposite position of the shift valve SFV, the reduction brake RD/B is allowed to communicate with a drain port and released, while the line pressure P.sub.L is allowed to cause engagement of the direct clutch D/C, conditioning the auxiliary gearing in the high gear position. With the one-way orifice OWO1 and accumulator ACC1, a build-up of hydraulic pressure applied to the reduction brake RD/B is controlled, while the one-way orifice OWO2 and accumulator ACC2 control a build-up of hydraulic pressure applied to the direct clutch D/C. Accordingly, the auxiliary gearing is shiftable between the low and high gear positions without any substantial shock.
As mentioned before, the one-way clutch is arranged in parallel to the reduction brake RD/B to complement the action of the reduction brake RD/B. Engagement timing of the reduction brake RD/B is hereinafter considered.
Assuming that, during engine drive forward running, the auxiliary gearing shifts from the high gear position to the low gear position owing to engagement of the reduction brake after releasing the direct clutch. Since the one-way clutch serves as a substitute for the reduction brake during engine drive forward running, it is required that the reduction brake is engaged upon expiration of a considerable time after the direct clutch has been released for the purpose of preventing interlock of the auxiliary gearing. If the auxiliary gearing interlocks, a great shock is induced.
Assuming that, during engine brake forward running or reverse running,. the auxiliary gearing shifts from the high gear position to the low gear position. Since, in this situation, the one-way clutch is not operable, it is required that the reduction brake is engaged immediately after the direct clutch has been released.
The above-mentioned two requirements have to be compromised in the conventional hydraulic control system. Referring again to FIG. 4, the pressure build-up in the reduction brake RD/B is always controlled by the one-way orifice OWO1 and accumulator ACC1. Thus, it is impossible to set two different engagement timings of the reduction brake RD/B which are suitable during engine drive forward running and during engine braking, respectively. If the priority is placed on engine braking performance, a relatively great shock is induced when the auxiliary gearing shifts from the high gear position to the low gear position during engine drive forward running. On the contrary, if the priority is placed on the engine drive forward running performance, a poor engine braking performance results. Thus, it is difficult to compromise between the above-two requirements.
An object of the present invention is to improve an automatic transmission such that the above-mentioned two requirements are met.